Rigging has a weird reaction on spheres
For whatever reasons, hard to say without knowing how you weighted the model, those vertices have not been properly weighted.  Thankfully, it's easy enough to fix.  Select your object and switch to weight paint mode.  Select the appropriate bone.  'v' switch to vertex mode and select the vertex (probably a single vert, so just RMB.)  In properties/data/vertex groups, make sure your assign scale is set to 1.0 and hit the assign button.  (Selecting the bone earlier should select the appropriate vertex group automatically.)

There are plenty of other ways to do this, some of which would be easier, depending on how your model is laid out.